1. Field of the Invention
This invention relates to a method for adjusting a loudness of an announcement sound signal relative to a loudness of an entertainment sound signal. The invention finds especially, but not exclusively, application in vehicles in which sound systems are present emitting an entertainment sound signal. Furthermore, a main control unit may be provided emitting an announcement signal, e.g., of a navigation system indicating the route to the driver.
2. Related Art
Motor vehicle navigation systems are known in the art and are proliferating. A navigation system calculates a route from the present location to a destination and shows on a display the next maneuvers to be taken by the driver. Additionally, it is possible that the driver is supported by an announcement indicating to the driver how to behave at an upcoming location. By way of example, the navigation system may alert the driver to turn right at the next corner.
In addition to this announcement sound signal, another sound source, such as a radio or a compact disc, a DVD or other digitally stored music stored as an MP3 file etc. might be utilized in the vehicle during driving. In case of an announcement by the navigation system, the announcement sound signal is output together with the sound signal of the other sound source, which, for purposes of this application, shall be referred to as an “entertainment sound signal.” In this situation, it may happen that the sound level of the announcement sound signal relative to the sound level of the entertainment sound signal is chosen in such a way that the content of the announcement may not be correctly understood by the driver as the signal level difference between the two signals may be too low.
In EP 1 821 572, a system for adapting the loudness of the entertainment sound signal relative to the announcement sound signal is disclosed. In this prior art, differentiation between the sound event in the physical domain and the auditory event relating to the human perception is made.
The sound pressure level (SPL) is a physical measure and is mathematically converted to decibel (dB). For an orally adequate rating, a weighting function is introduced. Sound events are perceived by humans not proportional to their physical sound pressure level. Depending on temporal and spectral masking, frequency and overall sound pressure level, the loudness of the auditory event differs from the SPL of the sound event. In EP 1 821 572, a fixed announcement gain offset is calculated, this announcement gain offset depends on the volume level set by the user and on a mean loudness determined by the system. This gain offset is a fixed value.
The auditory perception of a human being, however, depends on the age and on the individual function of the hearing. The auditory perception differs to a great extend for different users and is normally not linear. By way of example, if one's hearing is damaged, a small loudness difference is difficult to perceive, whereas above certain thresholds the damaged person's hearing corresponds to the hearing of a person with normal hearing. Additionally, the surrounding noise such as the noise of a motor in a vehicle, the noise of the tires, the noise of an open window etc. change the binaural intelligibility level difference (BILD). As a consequence, the auditory perception strongly depends on the user utilizing the audio source outputting the entertainment sound signal and the announcement sound signal. Accordingly, a need exists to improve the loudness perception based upon different hearing capabilities of a user of the audio system.